Postgresql 为什么 PG：：UniqueViolation： ERROR： duplicate key value violates unique constraint

我有一个模型Post，每次创建帖子时，我都希望同时创建Moderation的一个新实例.

所以在邮报上.rb我使用回调after_save :create_moderation

 ...
 include Reportable
 after_save :create_moderation

 private
 def create_moderation
    self.create_moderation!(blog: Blog.first)
 end

但是当一个提议被创建时，我会遇到这样的错误:

PG::UniqueViolation:错误:重复键值违反了唯一约束"可报告"详细信息:键(可报告类型，可报告id)=(Post，25)已存在:将($1，$2，$3，$4，$5，$6)返回"id"值插入"审核"("博客id"，"可报告类型"，"可报告类型"，"创建日期"，"更新日期"，"博客类型")

以可报告的形式.rb我有:

  has_one :moderation, as: :reportable, foreign_key: "reportable_id", foreign_type: "reportable_type", class_name: "Moderation"

然后是其他几个可报告对象的方法.

注意，当我在控制台中运行create方法时，这个问题不会发生.

EDIT

  create_table "moderations", id: :serial, force: :cascade do |t|
    t.string "reportable_type", null: false
    t.string "reportable_id", null: false
    t.integer "blog_id", null: false
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.string "blog_type", null: false
    t.string "upstream_moderation", default: "unmoderate"
    t.index ["blog_id", "blog_type"], name: "moderations_blog"
    t.index ["reportable_type", "reportable_id"], name: "moderations_reportable", unique: true
  end



create_table "posts", id: :serial, force: :cascade do |t|
    t.text "title", null: false
    t.text "body", null: false
    t.integer "feature_id", null: false
    t.integer "author_id"
    t.integer "scope_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "post_votes_count", default: 0, null: false
    t.index ["body"], name: "post_body_search"
    t.index ["created_at"], name: "index_posts_on_created_at"
    t.index ["author_id"], name: "index_posts_on_author_id"
    t.index ["feature_id"], name: "index_posts_on_feature_id"
    t.index ["proposal_votes_count"], name: "index_posts_on_post_votes_count"
    t.index ["title"], name: "post_title_search"
  end